Question

The decreasing order of the strength of bases \(\mathrm{OH}\), \(\mathrm{NH}_{2}, \mathrm{HC} \equiv \mathrm{C}\). and \(\mathrm{CH}_{3} \mathrm{CH}_{2}\) (1) \(\mathrm{CH}_{3} \mathrm{CH}_{2}>\mathrm{NH}_{2}>\mathrm{HC} \equiv \mathrm{C}>\mathrm{OH}\) (2) \(\mathrm{HC} \equiv \mathrm{C}>\mathrm{CH}_{3} \mathrm{CH}_{2}>\mathrm{NH}_{2}>\mathrm{OH}\) (3) \(\mathrm{OH}>\mathrm{NH}_{2}>\mathrm{HC} \equiv \mathrm{C}>\mathrm{CH}_{3}-\mathrm{CH}_{2}\) (4) \(\mathrm{NH}_{2}>\mathrm{HC} \equiv \mathrm{C}>\mathrm{OH}>\mathrm{CH}_{3}-\mathrm{CH}_{2}\)

Short answer

Option 1: \( \text{CH}_{3} \text{CH}_{2} > \text{NH}_{2} > \text{HC} \equiv \text{C} > \text{OH} \).

Step-by-step solution

Understand the problem

We need to arrange the bases \(\text{OH}\), \(\text{NH}_{2}\), \( \text{HC} \equiv \text{C}\), and \(\text{CH}_{3} \text{CH}_{2}\) in decreasing order of their base strength and then find out which of the given options is correct.

Knowing the strength of bases

The strength of a base is determined by its ability to donate a pair of electrons. Electron-donating groups increase the basicity of a molecule. Let's consider the given bases: \( \text{CH}_{3} \text{CH}_{2}\) (ethyl anion), \( \text{NH}_{2}\) (amide anion), \( \text{HC} \equiv \text{C}\) (acetylide anion), and \( \text{OH}\) (hydroxide anion).

Compare each anion's capacity to donate electrons

Among the given anions, the ethyl anion \( \text{CH}_{3} \text{CH}_{2}\) is the strongest because alkyl groups are strong electron-donating groups. Next, the amide anion \( \text{NH}_{2}\) is strongly basic as well. The acetylide anion \( \text{HC} \equiv \text{C}\) has less electron-donating capability compared to \( \text{NH}_{2}\). Lastly, the hydroxide anion \( \text{OH}\) is the weakest base in this group.

Arrange the bases in decreasing order

From the analysis, the decreasing order of their base strength is: \( \text{CH}_{3} \text{CH}_{2} > \text{NH}_{2} > \text{HC} \equiv \text{C} > \text{OH} \).

Match the order with given options

Looking at the given options:1. \( \text{CH}_{3} \text{CH}_{2} > \text{NH}_{2} > \text{HC} \equiv \text{C} > \text{OH} \)2. \( \text{HC} \equiv \text{C} > \text{CH}_{3} \text{CH}_{2} > \text{NH}_{2} > \text{OH} \)3. \( \text{OH} > \text{NH}_{2} > \text{HC} \equiv \text{C} > \text{CH}_{3} \text{CH}_{2} \)4. \( \text{NH}_{2} > \text{HC} \equiv \text{C} > \text{OH} > \text{CH}_{3} \text{CH}_{2} \)Option 1 matches our determined order.

Key concepts

electron-donating groups

Electron-donating groups (EDGs) are substituents that have the ability to donate electron density towards a molecule's central atom. They play a crucial role in the basicity of a molecule.

For instance, consider an alkyl group (like in the ethyl anion, \(\text{CH}_3\text{CH}_2\)). Alkyl groups are known to be strong electron donors. They make the molecule more basic because they increase the electron density around the atom that carries the negative charge.

This boost in electron density enhances the molecule's ability to donate a lone pair of electrons, making it a stronger base. Therefore, bases that have strong electron-donating groups are typically more basic compared to those with weaker or electron-withdrawing groups.

base strength comparison

Base strength is fundamentally related to a molecule's propensity to donate a pair of electrons. Let's compare the base strengths of the anions given in the exercise: \(\text{OH}\), \(\text{NH}_2\), \(\text{HC} \equiv \text{C}\), and \(\text{CH}_3\text{CH}_2\).

The ethyl anion (\(\text{CH}_3\text{CH}_2\)) is the strongest base in this group. This is because alkyl groups are strong electron-donating groups, and they increase the electron density on the central atom. This increased electron density makes the donation of electrons (basicity) more favorable.

Next comes the amide anion (\(\text{NH}_2\)). Though less strong than the ethyl anion, it is still quite a strong base because nitrogen has a lone pair of electrons that contributes significantly to basicity.

The acetylide anion (\(\text{HC} \equiv \text{C}\)) is less basic compared to the ethyl and amide anions. The triple bond between the carbon atoms withdraws electron density, making it less willing to donate electrons.

Lastly, the hydroxide anion (\(\text{OH}\)) is the weakest base in this set. Oxygen is more electronegative and holds onto its electrons more tightly, which means it is less able to donate them compared to the other bases in consideration.

anion electron donation

Anions act as bases by donating a pair of electrons. Here, we'll dive into how particular anions donate electrons and how this influences their basicity.

Let's start with the ethyl anion (\(\text{CH}_3\text{CH}_2\)). This anion benefits from the electron-donating ability of the alkyl group, making it highly favorable for electron donation.

The amide anion (\(\text{NH}_2\)) is also highly effective at donating electrons due to the nitrogen atom's lone pair. This makes it a strong base, although not as strong as the ethyl anion.

The acetylide anion (\(\text{HC} \equiv \text{C}\)), with its triple-bond structure, is less capable of donating electrons. The electron density is heavily centered around the triple bond, making it less readily available for donation.

Finally, the hydroxide anion (\(\text{OH}\)) is the weakest base in this group due to the high electronegativity of oxygen. Oxygen holds onto its electrons more tightly, making it less likely to donate them compared to the other anions discussed.

Thus, the electron-donating capability of anions significantly influences their basicity, with stronger electron donors generally resulting in stronger bases.